1. Field of the Invention
The present invention falls within the technical field that relates to an ink composition for inkjet recording, an ink cartridge and a recording apparatus which are suitable for inkjet recording.
2. Description of the Prior Art
Conventionally, ink containing a colorant (dye or pigment), a humectant and water has been well known as ink used for inkjet recording. However, in the case of forming an image with the ink on a recording medium, the water-resistivity of the image is a matter of concern, i.e., there is a problem such that the colorant exudes into water when the image is exposed to water. Especially when an image is recorded on plain paper, the water-resistivity of the image is very poor. (The “plain paper” herein refers to paper which is one of various types of commercially-available paper, which is especially used for an electrophotographic copying machine, and which is produced without an intention to have an optimum structure, composition, properties, or the like, for inkjet recording.)
In Japanese Unexamined Patent Publication No. 10-212439, Japanese Unexamined Patent Publication No. 11-293167, Japanese Unexamined Patent Publication No. 11-315231, and Japanese Unexamined Patent Publication No. 2000-178494, adding a hydrolyzable silane compound (organic silicon compound) to ink in order to improve the water-resistivity of an image formed with the ink on a recording medium has been proposed. When a drop of such ink containing a silane compound is adhered on a recording medium, and a water content (solvent) of the ink drop evaporates or permeates into the recording medium, the silane compound remaining on the recording medium is condensation-polymerized, and this condensation-polymerized silane compound encloses a colorant. As a result, even when the image formed on the recording medium is exposed to water, the colorant is prevented from exuding into the water.
The water-resistant ink disclosed in each of the above publications provides a beneficial effect-of improving the water-resistivity. However, if an image formed with such water-resistant ink is stored for a long time, the color of the image fades away. It is estimated that this problem is caused by the following mechanism. When the image formed with the water-resistant ink on the recording medium is exposed to light (ultraviolet light) from the sun or a fluorescent lamp, the energy of the light attacks the azo group (—N═N—) of the colorant. As a result, a double bond of the azo group is changed into a single bond, or the double bond is cleaved, whereby the structure of the colorant is decomposed. The decomposition of the colorant structure causes a change in hue of the image and/or a decrease in color depth of the image (i.e., fading of color of the image).
Furthermore, in the case where an image formed on a recording medium with the water-resistant ink disclosed in the above publications is stored for a long time, the color of the image fades away even in the case where it is not exposed to ultraviolet light. It is estimated that this problem is caused by the following mechanism. The azo group (—N═N—) of the colorant is attacked by peroxyradical or ozone which is generated during the process of thermal oxidation caused by heat, moisture, and/or a chemical substance, etc., whereby the structure of the colorant is decomposed.
For example, Japanese Unexamined Patent Publication No. 2001-240778 proposes adding thiocyanate to commonly-employed ink which does not have water-resistivity (i.e., ink not containing an organic silicon compound) in order to improve the ozone-resistance of the ink. Such a proposition suggests adding the thiocyanate to the above water-resistant ink for the purpose of improving the weatherability of the ink.
However, it was confirmed that only adding thiocyanate to the water-resistant ink does not much improve the weatherability of an image formed with the ink. Furthermore, it was also confirmed that if the image formed with the ink is once exposed to water, the weatherability of the image significantly deteriorates.